Use of nanomaterials as effective viscosity modifiers in lubricating fluids

ABSTRACT

Nanomaterials have been used as a supplement or replacement of traditional polymer-based viscosity modifiers for lubricants and other related fluids. Compared with traditional polymer-based viscosity modifiers, nanomaterials possess better viscosity-index modification functions, i.e., more even viscosity increase across the whole temperature range. Meanwhile, a cost-effective way of making nanomaterials have been developed based on commercially available graphite materials, and the resulting nanoparticles of graphite are nanodisks (nanoplates). Furthermore, it provides a viscosity modifier which exhibits temporary shear loss, which can contribute to fuel economy, but no permanent shear loss.

This application claims priority from U.S. Provisional Application Ser.No. 60/592,570 filed on Jul. 31, 2004 and Provisional Application Ser.No. 60/254,959 filed on Dec. 12, 2000 and U.S. Pat. No. 6,783,746 whichissued on Aug. 31, 2004 from application Ser. No. 10/021,767 filed onDec. 12, 2001 and application Ser. No. 10/929,636 filed on Aug. 30, 2004and application Ser. No. 10/730,762 filed on Dec. 8, 2003 which claimspriority from PCT/US02/16888 filed on May 30, 2002 and application Ser.No. 10/737,731 filed on Dec. 16, 2003 all of which are incorporatedherein in their entirety.

TECHNICAL FIELD

The present invention relates to a novel use of nanomaterial as aviscosity modifier for a lubricating oil and a lubricating oilcomposition. More particularly, the invention relates to a novelviscosity modifier for a lubricating oil capable of producing alubricating oil composition having excellent viscosity index andlubricating oil compositions containing such a viscosity modifier.

BACKGROUND OF THE INVENTION

The viscosity of petroleum products generally varies greatly withtemperature, and for lubricating oils for automobiles, the temperaturedependence of the viscosity is desired to be small. Therefore, a polymerhas been widely used as a viscosity modifier having an effect ofimproving viscosity index for the purpose of decreasing the temperaturedependence of the lubricating oils.

Viscosity index of a fluid is defined as the relationship of viscosityof that fluid to the temperature. It is determined by measuring thekinematic viscosities of the oil at 40 and 100° C. and then calculatedby using the tables or formulas included in ASTM D 2270. High viscosityindex fluids (e.g., a base oil with the addition of a viscositymodifier) tend to display less change in viscosity with temperature thanlow viscosity index fluids (e.g., that base oil), and the effect isillustrated in FIG. 1.

Mineral oils, which are very effective lubricants at low temperatures,become less effective lubricants at high temperatures. At hightemperatures, their film-forming ability (in the hydrodynamiclubrication regime) diminishes, because of a drop in viscosity. Prior tothe use of viscosity modifiers and the introduction of multigrade oils,this problem was partly overcome through seasonal oil changes. Theprincipal function of a viscosity modifier is to minimize viscosityvariations with temperature. Viscosity modifiers are typically added toa low-viscosity oil to improve its high-temperature lubricatingcharacteristics. These are organic polymers that minimize viscositychange with a change in temperature. This represents a practical meansby which the operating range of mineral oils is extended to hightemperature without adversely affecting their low-temperature fluidity.The mechanism is explained as follows.

At low temperature, the polymer molecules occupy a small volume(hydrodynamic volume) and therefore have a minimum association with thebulk oil. The effect should be little viscosity increase. The situationis reversed at high temperature because polymer chains extend or expandas a consequence of added thermal energy. This increases the associationof the polymer with bulk oil because of an increase in surface area. Theresult is an effective increase in viscosity at this high temperature.FIG. 2 illustrates oil thickening by viscosity modifiers.

Olefin copolymers (OCP), polymethacrylates (PMA), hydrogenatedstyrene-diene (STD), and styrene-polyester (STPE) polymers are the mostcommon types of viscosity modifiers used in modern lubricantformulations.

However, there is always some undesired viscosity increase at lowtemperature (under the operating temperature range) caused by theseviscosity modifiers. That is, the viscosity index improvement by thepolymers is limited. Moreover, these polymers contribute to a higherextreme-low-temperature viscosity and wax formulation.

When the surrounding temperature lowers, a wax component in alubricating oil is crystallized and solidified to make the lubricatingoil lose flowability, so a pour point depressant (PPD) is usually addedinto the lubricating oil to depress the solidification temperature. Thepour point depressant functions to inhibit formation of athree-dimensional network attributed to crystallization of the waxcomponent in the lubricating oil and to depress the pour point of thelubricating oil. Of the low-temperature properties of a lubricating oilcontaining a viscosity modifier, having an effect of improving viscosityindex, and a pour point depressant, the viscosity at a high shear rateis determined by compatibility of a lubricating oil base with theviscosity modifier, but on the other hand, the viscosity at a low shearrate is greatly influenced by the pour point depressant. It is knownthat when an ethylene/α-olefin copolymer having specific composition isused as a viscosity modifier, the effect of the pour point depressant ismarkedly reduced because of an interaction between the copolymer and thepour point depressant (e.g., U.S. Pat. No. 3,697,429 and U.S. Pat. No.3,551,336). Accordingly, the viscosity modifier to be blended with alubricating oil which is required to have particularly excellentlow-temperature properties is desired to exhibit an excellent effect ofimproving viscosity index and at the same time not to inhibit thefunction of the pour point depressant.

The thickening effect of particles to a fluid base is well known (P. C.Hiemenz and R. Rajagopalan, Principles of Colloid and Surface Chemistry,3^(rd) ed., Marcel Dekker, Inc., 1997, Chapter 4). The initial theory ofexplaining this was developed by Albert Einstein in 1906, and there havebeen various modifications and deviations in this theory, and thedetails of which are obviously out of the scope of the currentinvention. Nanoparticles have been added to a fluid for the purpose ofincreasing thermal conductivity (U.S. Pat. No. 6,221,275, U.S. Pat. No.6,432,320, and U.S. Pat. No. 6,695,974). However, there has been littleor no effort in addressing the issue of viscous thickening effect ofthese nanoparticles. In most of the cases this viscous thickening effectis undesirable, since the increased viscosity will results in moredemand for pumping power, more energy loss due to internal fluidfriction, and even malfunction or catastrophic failure of the machineryif the viscosity is way off the desired range.

However, in the current invention, with very careful formulation, theviscous thickening effect of the nanoparticles could be turned into anapplication as a revolutionary viscosity modifier. And because thenanoparticles are usually not polymer based, they are not going to causecompatibility issue with other polymeric additives/components in alubricating fluid, and they are usually not contributing to waxformation by themselves.

It is common understanding in the lubricant industry that thinner fluidmay provide better fuel economy if adequate film thickness is properlymaintained. The reason is that the energy loss due to internal frictionof fluid itself is less when the viscosity is lower. Therefore, if theviscosity modifier can be sheared down temporarily (but notpermanently), fuel economy benefit could be observed. In the event ofcurrent invention, the nanodisks (or nanoplates) orient themselves in alaminar flow regime (Literature cited: Y. Yang, E. Grulke, Z. Zhang, G.Wu, Rheological Behavior of Carbon Nanotube and Graphite Dispersions,submitted to Langmuir), which indicates that temporary shear loss willbe observed should the fluid be place in a shear field.

SUMMARY OF THE INVENTION

In this invention, the use of nanoparticles as an effective viscositymodifier is illustrated. More specifically the use of carbonnanomaterial will be addressed. More specifically, cost-effectivegraphite materials and the process of making them into nanoparticleswill be illustrated.

The use of nanoparticles in a fluid base is well know, as illustrated bythe previous US Patents. The use of graphite in fluids such aslubricants is also well known. The graphite is added as a frictionreducing agent, which also carries some of the load imposed on theworking fluid, and therefore helps to reduce surface damage to workingparts. In order to be low friction, it is well known that the graphitelayered structure must contain some water or other material to createthe interlayer spacing and thereby lamellar structure. There are variouscommercially available graphite suspensions, e.g., from Acheson ColloidCo., which are specifically intended for use in lubricants. The size ofthe particles is varied for different dispersions, but the minimumaverage size for commercially available products is in the submicronrange or larger, typically mean as 500-800 nm. The viscositymodification advantage of the graphite is not mentioned in the salesliterature, nor is the product sold or promoted for its viscositymodification property.

While there have been various patents filed on lubricants containinggraphite, e.g. U.S. Pat. No. 6,169,059, there are none whichspecifically rely on graphite to improve the viscosity index of thefluid. Furthermore, there are none which teach specifically the use ofnanometer-sized graphite with mean particle size much significantly lessthan 1000 nm in order to increase viscosity index. Whilegraphite-containing automotive engine oil was once commercialized (Arcographite), the potential to use graphite as a viscosity modifyingmaterial in this oil was not realized. The particle size of graphiteused was larger (mean greater than one micron) than for the instntinvention as shown in FIG. 3. As a result, the graphite had somesettling tendency in the fluid. Graphite of this size also significantlyaffects the friction and wear properties of the fluid, and heretoforehas been used to reduce friction and improve wear performance of thefluid, e.g. in metalworking fluids. On the other hand, the use ofgraphite in lubricants for recirculating systems was made unpopular,partly due to evidence that micron size graphite could “pile up” inrestricted flow areas in concentrated contacts, thereby leading tolubricant starvation. No recognition of effect of graphite particle sizeon this phenomena was made.

Previously, naturally formed “nano-graphites” have not been available inthe marketplace at all. Recently, Hyperion Catalysis International, Inc.commercialized carbon nanotubes or so-called carbon fibrils, which havea graphitic content, e.g., U.S. Pat. No. 5,165,909. Carbon nanotubes aretypically hollow graphite-like tubules having a diameter of generallyseveral to several tens nanometers. They exist in the form either asdiscrete fibers or aggregate particles of nanofibers.

Bulk graphite is available from POCO Graphite as a graphite foam, and isalso available from the Carbide/Graphite Group, Inc. Graphite powderscan be obtained from UCAR Carbon Company Inc., and from Cytec CarbonFibers LLC. These bulk or powdery materials must be reduced to ananometer-sized particles by various methods for use in the instantinvention.

In this invention, fluids of enhanced viscosity index are prepared bydispersing nanometer-sized particles, especially carbon nanomaterials,into the fluid. The term carbon nanomaterials used in this inventionrefers to graphite nanoparticles, carbon nanotubes or fibrils, and othernanoparticles of carbon with graphitic structure. Stable dispersion isachieved by physical and chemical treatments.

The present invention provides at a minimum, a fluid of lubricantcontaining from 0.001% to 50% by weight nanoparticles, and preferably,from 0.01% to 25% by weight, and more preferably, from 0.1% to 20% byweight of nanoparticles. Preferably, however, a minimum of one or morechemical dispersing agents and/or surfactants are also added to achievelong-term stability. The term “dispersant” in the instant inventionrefers to a surfactant added to a medium to promote uniform suspensionof extremely fine solid particles, often of colloidal size. In thelubricant industry the term “dispersant” is generally accepted todescribe the long chain oil soluble or dispersible compounds whichfunction to disperse the “cold sludge” formed in engines. The term“surfactanf” in the instant invention refers to any chemical compoundthat reduces surface tension of a liquid when dissolved into it, orreduces interfacial tension between two liquids or between a liquid anda solid. It is usually, but not exclusively, a long chain moleculecomprised of two moieties: a hydrophilic moiety and a lipophilic moiety.The hydrophilic and lipophilic moieties refer to the segment in themolecule with affinity for water, and that with affinity for oil,respectively. These two terms, dispersant and surfactant, are mostlyused interchangeably in the instant invention. The particle-containingfluid of the instant invention will have a viscosity index higher thanthe conventional fluid of the same type. The fluid can have any otherchemical agents or other type particles added to it as well to impartother desired properties, e.g. friction reducing agents, antiwear oranticorrosion agents, detergents, antioxidants, dispersants, or thermalproperty booster. Furthermore, the term fluid in the instant inventionis broadly defined to include pastes, gels, greases, and liquidcrystalline phases in either organic or aqueous media, emulsions andmicroemulsions.

As set forth above, the nanomaterial could be of any commerciallyavailable nanoparticles, or any material which can be wet-milled intonanometer-sized particles using the process developed in this inventionwhich will be explained in detail later. One of the preferablenanoparticles are carbon-based materials. A preferred form of carbonnanomaterials is carbon nanotubes. Another preferred form of carbonnanomaterials is graphite. A preferred form of graphite is POCO Foamfrom POCO Graphite. Another preferred form is graphite powders from UCARCarbon Company Inc. Still another preferred form of graphite is graphitepowders from Cytec Carbon Fibers LLC. Still another preferred form ofgraphite is bulk graphite from The Carbide/Graphite Group, Inc. Anotherpreferable nanomaterial is aluminum oxide nanoparticles from Sasol.

The nanoparticle containing dispersion may also contain a large amountof one or more other chemical compounds, such as polymers, antiwearagents, friction reducing agents, anti-corrosion agents, detergents,metal passivating agents, antioxidants, antifoaming agents, corrosioninhibitors, pour point depressants, and additional conventionalpolymer-based viscosity improvers.

Furthermore, the nanomaterial dispersion can be pre-sheared, in aturbulent flow, such as a nozzle, or a high pressure fuel injector, aultrasonic device, or a mill in order to achieve a stable viscosity.This may be especially desirable in the case where carbon nanotubes withhigh aspect ratio are used as the graphite source, since they, even morethan spherical particles, will thicken the fluid but loose viscositywhen exposed in turbulent flows such as the flow regime in engines.Pre-shearing, e.g. by milling, sonicating, or passing through a smallorifice, such as in a fuel injector, is a particularly effective way todisperse the particles and to bring them to a stable size so that theirviscosity modifying effect will not change upon further use in actualapplications.

The milling process itself, or other pre-shearing process, can have arather dramatic effect on the long term dispersion stability. It hasbeen found that a preferred process is to mill the particles to a thickpasty liquid of particles with mean size less than 500 nanometers indiameter. The pasty liquid is then used as concentrate to preparelubricants of various viscosity grades, and can be easily diluted tomake a fluid with suitable viscosity for an desired application as anautomotive fluid, i.e., engine oil, automatic transmission fluid, gearoil, shock absorber oil, etc. A very effective paste can be made bymixing particles in a viscous base fluid in a loading of 5% to 20% byweight and milling for a period of several hours. The base fluidpreferably contains from 0% up to 100% of the dispersant/surfactantmixture with the remainder being natural, synthetic, or mineral baseoil. Once the concentrate prepared by milling is diluted to liquidconsistency with base oil and other lubricating fluid components, theentire fluid can (optionally) be passed through a small orifice deviceto further increase the uniformity and decrease the size of dispersedparticles.

An important aspect of this invention is that the final lubricant shouldbe prepared to give an acceptable lubricant film thickness at themaximum shear rate and temperature of use in the target application. Themaximum concentration of particles in the final (diluted) lubricatingfluid is limited by the relationship between viscosity increase of thefluid caused by the particles, and the temporary loss of viscosity(associated with the particles) at maximum temperature and shear rate offluid use. In general, the viscosity of the lubricant of the instantinvention will be matched with conventional fluid at high operatingtemperature, typically, 100° C., and the lower temperature viscosity ofthe lubricant of the instant invention will be lower than that of theconventional fluid. This means that the viscosity index of theparticle-containing lubricant of the instant invention will be higherthan that of the conventional fluid.

It is an object of the present invention to provide a viscosity modifierfor a lubricating oil, which provides better viscosity index, and withno adverse effect to the low temperature properties of the fluid, thanthe currently used polymer-based viscosity modifiers.

It is an object of the present invention to provide a cost-effectivematerial as a supplement or replacement for the conventionalpolymer-based viscosity modifiers.

It is an object of the present invention to development a cost-effectiveprocessing method for making the nanomaterial to be used as viscositymodifiers in lubricating oils.

It is an object of the present invention to use the cost-effectivegraphite as the source of the nanomaterials to be used as viscositymodifiers in lubricating oils.

It is an object of the present invention to provide a viscosity modifierwhich exhibits temporary shear loss, which will contribute to fueleconomy upon use in a motor vehicle, but no permanent shear loss.

It is an object of the present invention to provide a method ofpreparing a lubricant as a stable dispersion of the carbon nanomaterialsin a liquid medium with the combined use of dispersants/surfactants andphysical agitation.

It is an object of the present invention to provide a in which thecarbon nanomaterials are made from cost-effectivehigh-thermal-conductivity graphite (with thermal conductivity higherthan 80 W/m·K).

It is an object of the present invention to provide a method ofdeveloping a method of forming carbon nanomaterials from inexpensivebulk graphite.

It is an object of the present invention to provide a method ofutilizing carbon nanotube, graphite flakes, carbon fibrils, carbonparticles and combinations thereof.

It is an object of the present invention whereby the carbon nanomaterialcan optionally be surface treated to be hydrophilic at surface for easeof dispersing into the aqueous medium.

It is an object of the present invention to provide a method wherein thesaid dispersants/surfactants are soluble or highly dispersible in thesaid liquid medium.

It is an object of the present invention to provide a process forpreparing a lubricant composition containing nanomaterial by a)dissolving the said dispersants/surfactants or dispersant additivepackage into the base fluid; b) adding a high concentration (5-20% byweight) of the said carbon nanomaterials into the above mixture whilebeing strongly agitated by high impact milling, and/or ultrasonication,to form a pasty liquid; and c) the pasty liquid obtained in b) isfurther diluted with base oil and additives as needed to make the finallubricant.

It is an object of the present invention to provide a method of using aliquid medium selected from a natural oil (vegetable or animal oil), ora synthetic oil, or a mineral oil or a combination thereof.

It is an object of the present invention to provide a method of definingan appropriate dispersants/surfactants for a liquid medium of the typeused in the lubricant industry, whereby it is a surfactant or a mixtureof surfactants with low hydrophile-lipophile balance (HLB) value of 8 orless, preferably nonionic or mixture of nonionic and ionic surfactants.

It is an object of the present invention to dissolve a dispersantcontaining a surfactant a having a HLB of 8 or less in an amount of from0.001 to 30.0 percent by weight of a lubricant liquid medium forming adispersant liquid lubricant medium, adding nanomaterial having an aspectratio of from 500 to 5,000 in an amount of from 0.001 to 10.0 percent byweight into the dispersant liquid lubricant medium with agitation, andforming a uniform suspension of colloidal size solid particles ofnanomaterial having an enhanced thermal conductivity when compared tothe same lubricant medium containing no nanomaterial.

It is an object of the present invention to dissolve a dispersantcontaining a surfactant a having a HLB of 8 or less in an amount of from0.001 to 30.0 percent by weight of a lubricant liquid medium forming adispersant liquid lubricant medium, adding carbon nanomaterial having anaspect ratio of from 500 to 5,000 in an amount of from 0.001 to 10.0percent by weight into the dispersant liquid lubricant medium withagitation, and forming a uniform suspension of colloidal size solidparticles of carbon nanomaterial having an enhanced thermal conductivitywhen compared to the same lubricant medium containing no carbonnanomaterial.

It is an object of the present invention to provide that the dispersantscan be the ashless polymeric dispersants used in the lubricant industry.

It is an object of the present invention to provide a uniform dispersionin the form of a gel or paste with designed viscosity of carbonnanomaterials in base oil medium.

It is an object of the present invention to provide a uniform dispersionin a form as a gel or paste of high thermal conductivity graphitenanoparticle in petroleum, natural, or synthetic liquid medium.

It is an object of the present invention to provide a uniform dispersionin its final form as an automatic transmission fluid of relatively lowviscosity (kinematic viscosity less than 10 centistokes at (100° C.).

It is an object of the present invention to provide a uniform and stabledispersion in a form containing dissolved non-dispersing, otherfunctional compounds in the liquid medium.

It is an object of the present invention to provide a uniform and stabledispersion in a form without polymeric viscosity index improvers, whereall viscosity index improvement comes from the carbon nanomaterials.

It is an object of the present invention to provide a uniform and stabledispersion where due to the absence of polymeric materials thedispersion exhibits no permanent, only temporary loss in viscosity dueto shear fields and turbulence.

It is an object of the present invention to provide a uniform and stabledispersion where the carbon nanomaterials are used to convey anextremely high viscosity index, >200, and even >300.

It is an object of the present invention to provide a uniform and stabledispersion where the thermal conductivity and heat transfer capabilityof the fluid is at least more than 20% improved compared to conventionalmineral oil based automatic transmission fluids.

Other objects, features, and advantages of the invention will beapparent with the following detailed description taken in conjunctionwith the accompanying drawings showing a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had uponreference to the following description in conjunction with theaccompanying drawings in which like numerals refer to like partsthroughout the several views and wherein:

FIG. 1 is a graph showing the effect of viscosity modifiers improvingthe viscosity index (VI) of a base oil;

FIG. 2 shows the working mechanism of a polymer-based viscositymodifier;

FIG. 3 shows a scanning electron microscope photomicrograph of aconventional graphite-containing oil;

FIG. 4 shows a scanning electron microscope photomicrograph of anautomatic transmission fluid (ATF) oil sample containing the graphitenanodisks and platelets in a final automatic transmission fluidprocessed by the wet-milling method; and

FIG. 5 shows an atomic force microscope, (AFM) picture of the automatictransmission fluid of FIG. 4 wherein the grid size is 1×1 micron and theheight is 5 nm showing the ATF oil sample containing the graphitenanodisks and platelets in a final automatic transmission fluidprocessed by the wet-milling method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a nanomaterial-containing fluid mediumthat possesses higher viscosity index (smaller viscosity change tendencywith temperature) compared to conventional fluids of the same medium. Inthe present invention the fluid medium is targeted in its lubrication,viscosity, friction, antioxidant and thermal management characteristicsto perform in modern automotive machineries.

One of the preferred nanomaterials are carbon nanotubes. The nanotubescan be either single-walled, double-walled, or multi-walled, having atypical nanoscale diameter of 1-200 nanometers. More typically thediameter is around 10-30 nanometers. The length of the tube can be insubmicron and micron scale, usually from 50 nanometers to 100 microns.More typical length is 500 nanometers to 50 microns. The aspect ratio ofthe tube (which is defined by the average length of the tubes divided bythe average diameter) can be from hundreds to thousands, more typical100 to 2000. The surface of the nanotube can be treated chemically toachieve certain level of hydrophilicity, or left as is from theproduction.

Another preferred form of nanomaterials are commercially availablegraphite, e.g. POCO Foam, available from POCO Graphite, Inc., andgraphite powders available from UCAR Carbon Company Inc.

POCO Foam is a high thermal conductivity foamed graphite, thermalconductivity typically in the range 100 to 150 W/m·K. A readilycommercially available graphite is graphite powders from UCAR CarbonCompany Inc. Still another preferred nanomaterial is the high thermalconductivity bulk graphite, Part#875G, from The Carbide/Graphite Group,Inc. Either of these graphite is prepared for the instant invention bypulverizing to a fine powder, dispersing chemically and physically in afluid of choice, and then ball milled or otherwise size-reduced until aparticle size of less than 500 nm diameter mean size is attained.Graphite nanoparticles this small usually exhibit the morphology as“nanodisk” or “nanoplate”, i.e., disk-like or plate-like particles inthe nanometer-size scale, with average diameter much larger than theaverage thickness of particles. The preferred method is to disperse thegraphite by ball milling in a viscous fluid of certain additives(detergents, dispersants, etc.) and then diluting the obtainedconcentrate with base oil and other additives as needed to attain thefinal viscosity and performance characteristics. The finer the particlesize attained upon milling, the better the viscosity thickening effectof the pasty concentrate to the final blend. The viscous thickeningeffect must be carefully balanced to attain a suitable lubricating filmthickness at the maximum shear rate and temperature of fluid use. Ingeneral, any commercially available graphite material can be used,provided that pulverization, milling and other described chemical andphysical methods can be used to reduce the size of the final graphitedispersion to below a mean particle size of 500 nm (in diameter). FIG. 4and FIG. 5, respectively, show a scanning electron microscopic pictureand an atomic force microscopic picture of the graphitenanodisks/nanoplates in a final automatic transmission fluid processedby the wet-milling method.

Another preferred nanomaterial is aluminum oxide nanoparticles fromSasol North America. These are particles surface-treated to improvedispersability in fluid. Typical particle size is 25 nm.

In the process of making the lubricating fluid with the nanoparticles,the mechanical process and sequence of adding the components are crucialin order to fully take advantage of the high viscosity index of thenanoparticles and to make the final fluid product with exceptionallyhigh viscosity index. High impact mixing is necessary to achieve ahomogeneous dispersion. Ball mill is one of the examples of a highimpact mixer. In the instant invention, an Eiger Mini Mill (Model:M250-VSE-EXP) is used as the high impact ball mill. It utilizes highwear resistant zirconia beads as the grinding media and circulates thedispersion constantly during milling. To achieve the best milling effectand therefore the best viscosity index improvement, the proper millingprocedure has been developed. Firstly if the material is in bulk state,it must first be size reduced into powders (with average size less than100 microns). Then a 5% to 20% by weight of powder form of the material,and more preferably 10% by weight of the powders, in base oil dispersionis milled into a paste state. Usually this step takes about 3 to 4hours. Then add appropriate amount of dispersing agent(s), usually 1 to2 times of the weight of particle, into the mill. With the addition ofdispersing agent(s) the paste changes from paste into liquid almostinstantly, and extended milling becomes possible. For most cases theextended milling period is 4 hours. It should be pointed out that if themixture in the mill turns into a paste, the recirculation of it becomesvery difficult and thus a poor milling is resulted. It is also foundthat if the dispersing agent(s) is added into the mill at the verybeginning, the viscosity index of the final nanofluids made from themilling process is not as high.

Example of Making the Fluids

Graphite particles are obtained by pulverizing big graphite chunks fromThe Carbide/Graphite Group, and size-selected through a mesh filter tobe less than 75 μm. 30 grams of the above graphite particles and 270grams of DURASYN 162 (a commercial 2 centistokes polyalphaolefin,abbreviated hereafter as 2 cSt PAO) were added into the EIGER Mini Mill(Model: M250-VSE-EXP). The milling speed was gradually increased to 4000rpm. In about 4 hours the above mixture turned into thick paste.Discharged 60 grams of this paste and labeled it as Paste A. For therest of the mixture in the mill, added 48 grams of a dispersant andinhibitor package (DI package) from Lubrizol, LUBRIZOL 9677MX, into themill and the paste became very thin, and successful recirculationrestored. Stopped the mill after another 4 hours of milling and labeledthe discharged paste as Paste B. Paste C was obtained by milling amixture of 30 grams of graphite with diameter less than 75 μm, 60 gramsof LUBRIZOL 9677MX, and 270 grams of Durasyn 162 at 4000 rpm for 8hours. Note here the dispersing agent LUBRIZOL 9677MX was added into themill at the very beginning. Then we formulate three automatictransmission fluids A through C, using the above three pastes asconcentrate, and their final composition is exactly the same: 2%graphite, 4% LUBRIZOL 9677 MX, 18% DuraSyn 162, 76% Durasyn 166 (acommercial 6 centistokes polyalphaolefin, abbreviated hereafter as 6 cStPAO) (all percentage by weight). Example 1 illustrates the 100° C.viscosity and viscosity index (VI) of the fluids. It was also found thatthe graphite particle size before milling was very critical on theviscosity modification effect as well. For example, starting withgraphite smaller than 10 μm (obtained as graphite powder from UCARCarbon Company Inc.) and following the same procedure as Paste B, a thinPaste D was obtained.

Oil Basestocks

The petroleum liquid medium can be any petroleum distillates orsynthetic petroleum oils, greases, gels, or oil-soluble polymercomposition. More typically, it is the mineral basestocks or syntheticbasestocks used in the lube industry, e.g., Group I (solvent refinedmineral oils). Group II (hydrocracked mineral oils), Group III (severelyhydrocracked oils, sometimes described as synthetic or semi-syntheticoils), Group IV (PAOs), and Group V (esters, naphthenes, and others).One preferred group includes the polyalphaolefins, synthetic esters, andpolyalkylglycols.

Synthetic lubricating oils include hydrocarbon oils and halo-substitutedhydrocarbon oils such as polymerized and interpolymerized olefins (e.g.,polybutylenes, polypropylenes, propylene-isobutylene copolymers,chlorinated polybutylenes, poly(1-octenes), poly(1-decenes), etc., andmixtures thereof; alkylbenzenes (e.g., dodecylbenzenes,tetradecylbenzenes, dinonylbenzenes, di-(2-ethylhexyl)benzenes, etc.);polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.),alkylated diphenyl, ethers and alkylated diphenyl sulfides and thederivatives, analogs and homologs thereof and the like. Alkylene oxidepolymers and interpolymers and derivatives thereof where the terminalhydroxyl groups have been modified by esterification, etherification,etc. constitute another class of known synthetic oils.

Another suitable class of synthetic oils comprises the esters ofdicarboxylic acids (e.g., phtalic acid, succinic acid, alkyl succinicacids and alkenyl succinic acids, maleic acid, azelaic acid, subericacid, sebacic acid, fumaric acid, adipic acid, alkenyl malonic acids,etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol,dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol diethylene glycolmonoether, propylene glycol, etc.). Specific examples of these estersinclude dibutyl adipate, di(2-ethylhexyl) sebacate, di-hexyl fumarate,dioctyl sebacate, diisooctyl azelate, diisodecyl azealate, dioctylphthalate, didecyl phthalate, dicicosyl sebacate, the 2-ethylhexyldiester of linoleic acid dimer, the complex ester formed by reacting onemole of sebacic acid with two moles of tetraethylene glycol and twomoles of 2-ethylhexanoic acid, and the like.

Esters useful as synthetic oils also include those made from C₅ to C₁₂monocarboxylic acids and polyols and polyol ethers such as neopentylglycol, trimethylolpropane, pentaerythritol, dipentaerythritol,tripentaerythritol, etc. Other synthetic oils include liquid esters ofphosphorus-containing acids (e.g., tricresyl phosphate, trioctylphosphate, diethyl ester of decylphosphonic acid, etc.), polymerictetrahydrofurans and the like.

Preferred polyalphaolefins (PAO), include those sold by Mobil ChemicalCompany as SHF fluids, and those sold by Ethyl Corporation under thename ETHYLFLO, or ALBERMARLE. PAO's include the Ethyl-flow series byEthyl Corporation, “Albermarle Corporation,” including Ethyl-flow 162,164, 166, 168, and 174, having varying viscosity from about 2 to about460 centistokes.

MOBIL SHF-42 from Mobil Chemical Company, EMERY 3004 and 3006, andQuantum Chemical Company provide additional polyalphaolefins basestocks.For instance, EMERY 3004 polyalphaolefin has a viscosity of 3.86centistokes at 212° F. (100° C.) and 16.75 centistokes at 104° F. (40°C.). It has a viscosity index of 125 and a pour point of −98° F. and italso has a flash point of 432° F. and a fire point of 478° F. Moreover,EMERY 3006 polyalphaolefin has a viscosity of 5.88 centistokes at 212°F. and 31.22 centistokes at 104° F. It has a viscosity index of 135 anda pour point of −87° F. It also has a flash point of 464° F. and a firepoint of 514° F.

Additional satisfactory polyalphaolefins are those sold by Uniroyal Inc.under the brand SYNTON PAO-40, which is a 40 centistokespolyalphaolefin. Also useful are the ORONITE brand polyalphaolefinsmanufactured by Chevron Chemical Company.

It is contemplated that Gulf SYNFLUID 4 centistokes PAO, commerciallyavailable from Gulf Oil Chemicals Company, a subsidiary of ChevronCorporation, which is similar in many respects to EMERY 3004 may also beutilized herein. MOBIL SHF-41 PAO, commercially available from MobilChemical Corporation, is also similar in many respects to EMERY 3004.

Preferably the polyalphaolefins will have a viscosity in the range ofabout 2-100 centistokes at 100° C., with viscosity of 4 and 10centistokes being particularly preferred.

The most preferred synthetic base oil ester additives are polyolestersand diesters such as di-aliphatic diesters of alkyl carboxylic acidssuch as di-2-ethylhexylazelate, di-isodecyladipate, anddi-tridecyladipate, commercially available under the brand name EMERY2960 by Emery Chemicals, described in U.S. Pat. No. 4,859,352 toWaynick. Other suitable polyolesters are manufactured by Mobil Oil. MOBLpolyolester P43, M-045 containing two alcohols, and Hatco Corp. 2939 areparticularly preferred.

Diesters and other synthetic oils have been used as replacements ofmineral oil in fluid lubricants. Diesters have outstanding extreme lowtemperature flow properties and good resistance to oxidative breakdown.

The diester oil may include an aliphatic diester of a dicarboxylic acid,or the diester oil can comprise a dialkyl aliphatic diester of an alkyldicarboxylic acid, such as di-2-ethyl hexyl azelate, di-isodecylazelate, di-tridecyl azelate, di-isodecyl adipate, di-tridecyl adipate.For instance, Di-2-ethylhexyl azelate is commercially available underthe brand name of EMERY 2958 by Emery Chemicals.

Also useful are polyol esters such as EMERY 2935, 2936, and 2939 fromEmery Group of Henkel Corporation and Hatco 2352, 2962, 2925, 2938,2939, 2970, 3178, and 4322 polyol esters from Hatco Corporation,described in U.S. Pat. No. 5,344,579 to Ohtani et al., and MOBIL ester P24 from Mobil Chemical Company. Mobil esters such as made by reactingdicarboxylic acids, glycols, and either monobasic acids or monohydricalcohols like EMERY 2936 synthetic-lubricant base stocks from QuantumChemical Corporation and MOBIL P 24 from Mobil Chemical Company can beused. Polyol esters have good oxidation and hydrolytic stability. Thepolyol ester for use herein preferably has a pour point of about −100°C. or lower to −40° C. and a viscosity of about 2-460 centistokes at100° C.

Group III oils are often referred to as hydrogenated oil to be used asone of the preferred base oil components of the instant inventionproviding superior performance to conventional lubricating oils with noother synthetic oil base or mineral oil base.

A hydrogenated oil is a mineral oil subjected to hydrogenation orhydrocracking under special conditions to remove undesirable chemicalcompositions and impurities resulting in a mineral oil based oil havingsynthetic oil components and properties. Typically the hydrogenated oilis defined as a Group III petroleum based stock with a sulfur level lessthan 0.03, severely hydrotreatd and isodewaxed with saturates greaterthan or equal to 90 and a viscosity index of greater than or equal to120, and may optionally be utilized in amounts up to 90 percent byvolume, more preferably from 5.0 to 50 percent by volume and morepreferably from 20 to 40 percent by volume when used in combination witha synthetic or mineral oil.

The hydrogenated oil my be used as the preferred base oil component ofthe instant invention providing superior performance to conventionalmotor oils with no other synthetic oil base or mineral oil base. Whenused in combination with another conventional synthetic oil such asthose containing polyalphaolefins or esters, or when used in combinationwith a mineral oil, the hydrogenated oil may be present in an amount ofup to 95 percent by volume, more preferably from about 10 to 80 percentby volume, more preferably from 20 to 60 percent by volume and mostpreferably from 10 to 30 percent by volume of the base oil composition.

A Group I or II mineral oil basestock may be incorporated in the presentinvention as a portion of the concentrate or a basestock to which theconcentrate may be added. Preferred as mineral oil basestocks are theMarathon Ashland Petroleum (MAP) 325 Neutral defined as a solventrefined neutral having a Sabolt Universal viscosity of 325 SUS at 100°F. and MAP 100 Neutral defined as a solvent refined neutral having a Sabolt Universal viscosity of 100 SUS at 100° F., both manufactured bythe Marathon Ashland Petroleum.

Other acceptable petroleum-base fluid compositions include whitemineral, paraffinic and MVI naphthenic oils having the viscosity rangeof about 20-400 centistokes. Preferred white mineral oils include thoseavailable from Witco Corporation, Arco Chemical Company, PSI andPenreco. Preferred paraffinic oils include solvent neutral oilsavailable from Exxon Chemical Company, HVI neutral oils available fromShell Chemical Company, and solvent treated neutral oils available fromArco Chemical Company. Preferred MVI naphthenic oils include solventextracted coastal pale oils available from Exxon Chemical Company, MVIextracted/acid treated oils available from Shell Chemical Company, andnaphthenic oils sold under the names HYDROCAL and CALSOL by Calumet, anddescribed in U.S. Pat. No. 5,348,668 to Oldiges.

Finally, vegetable oils may also be utilizes as the liquid medium in theinstant invention. Soybean or rapeseed oil, particularly of the higholeic or mid oleic genetically engineered type, commercially availablefrom Archer Daniels Midland Company, are good examples of these oils.Soybean oil is of interest because it has a high thermal conductivityitself.

Dispersants Dispersants used in Lubricant Industry

Dispersants used in the lubricant industry are typically used todisperse the “cold sludge” formed in gasoline and diesel engines, whichcan be either “ashless dispersants”, or containing metal atoms. They canbe used in the instant invention since they are found to be an excellentdispersing agent for nanoparticles with graphitic structure of thisinvention. They are also needed to disperse wear debris and products oflubricant degradation within the moving parts housing of an automobile.

The ashless dispersants commonly used in the automotive industry containan lipophilic hydrocarbon group and a polar functional hydrophilicgroup. The polar functional group can be of the class of carboxylate,ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus,ester carboxyl, anhydride, or nitrile. The lipophilic group can beoligomeric or polymeric in nature, usually from 70 to 200 carbon atomsto ensure oil solubility. Hydrocarbon polymers treated with variousreagents to introduce polar functions include products prepared bytreating polyolefins such as polyisobutene first with maleic anhydride,or phosphorus sulfide or chloride, or by thermal treatment, and thenwith reagents such as polyamine, amine, ethylene oxide, etc.

Of these ashless dispersants the ones typically used in the petroleumindustry include N-substitued polyisobutenyl succinimides andsuccinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkylmethacrylate-dialkylaminoethyl methacrylate copolymers,alkylmethacrylate-polyethylene glycol methacrylate copolymers, andpolystearamides. Preferred oil-based dispersants that are most importantin the instant application include dispersants from the chemical classesof alkylsuccinimide, succinate esters, high molecular weight amines,Mannich base and phosphoric acid derivatives. Some specific examples arepolyisobutenyl succinimide-polyethylencpolyamine, polyisobutenylsuccinic ester, polyisobutenyl hydroxybenzyl-polyethylcncpolyamine,bis-hydroxypropyl phosphorate. Commercial dispersants suitable fortransmission fluid are for example, LUBRIZOL 890 (an ashless PIBsuccinimide), LUBRIZOL 6420 (a high molecular weight PIB succinimide),Ethyl Hitec 646 (a non-boronated PIB succinimide). The dispersant may becombined with other additives used in the lubricant industry to form a“dispersant-detergent (DI)” additive package for a lubricant, e.g.,LUBRIZOL™ 9677MX (used in transmission fluids), and the whole DI packagecan be used as dispersing agent for the nanoparticle dispersions.

Dispersants used in the lubricant industry are typically used todisperse the “cold sludge” formed in gasoline and diesel engines, whichcan be either “ashless dispersants”, or containing metal atoms. They canbe used in the instant invention since they have been found to be anexcellent dispersing agent for soot, an amorphous form of carbonparticles generated in the engine crankcase and incorporated with dirtand grease.

The ashless dispersants commonly used in the automotive industry containan lipophilic hydrocarbon group and a polar functional hydrophilicgroup. The polar functional group can be of the class of carboxylate,ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus,ester carboxyl, anhydride, or nitrile. The lipophilic group can beoligomeric or polymeric in nature, usually from 70 to 200 carbon atomsto ensure oil solubility. Hydrocarbon polymers treated with variousreagents to introduce polar functions include products prepared bytreating polyolefins such as polyisobutene first with maleic anhydride,or phosphorus sulfide or chloride, or by thermal treatment, and thenwith reagents such as polyamine, amine, ethylene oxide, etc.

Of these ashless dispersants the ones typically used in the petroleumindustry include N-substituted polyisobutenyl succinimides andsuccinates, allyl methacrylate-vinyl pyrrolidinone copolymers, alkylmethacrylate-dialkylaminoethyl methacrylate copolymers,alkylmethacrylate-polyethylene glycol methacrylate copolymers, andpolystearamides. Preferred oil-based dispersants that are most importantin the instant application include dispersants from the chemical classesof alkylsuccinimide, succinate esters, high molecular weight amines,Mannich base and phosphoric acid derivatives. Some specific examples arepolyisobutenyl succinimide-polyethylenepolyamine, polyisobutenylsuccinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine,bis-hydroxypropyl phosphorate. The dispersant may be combined with otheradditives used in the lubricant industry to form a “dispersant-detergent(DI)” additive package, e.g., LUBRIZOL™ 9802A, and the whole DI packagecan be used as dispersing agent for the nanostructure suspension.

For instance, LUBRIZOL 9802A is described in the technical brochure(MATERIAL SAFETY DATA SHEET No. 1922959-1232446-3384064) by The LubrizolCorporation in Wickliffe, Ohio and is hereby incorporated by reference.LUBRIZOL 9802A is described as a motor oil additive is believed tocontain as an active ingredient a zinc dithiophosphate and/or zincalkyldithiophosphate.

LUBRIZOL 4999 is described in its Technical Brochure (MATERIAL SAFETYDATA SHEET No. 1272553-1192556-3310026) by the Lubrizol Corporation inWickliffe, Ohio and is hereby incorporated by reference. LUBRIZOL 9802Ais described as a engine oil additive and contains as an activeingredient from 5 to 9.9 percent of a zinc alkyldithiophosphate.

LUBRIZOL 7720C in amounts of about 40% and LUBRIZOL 5186B in amounts ofup to about 1% are especially useful for shock absorber nanofluidscontaining nanostructures.

OLOA 9061 is described in Technical Brochure “MATERIAL SAFETY DATA SHEETNo. 006703” by Chevron Chemical Company LLC and is hereby incorporatedby reference. OLOA 9061 is described as zinc alkyl dithiophosphatecompound.

IGEPAL CO-630 is described in Technical Brochure “MATERIAL SAFETY DATASHEET” from Rhodia Inc. and is hereby incorporated by reference. IGEPALCO-630 is described as a nonylphenoxy poly(ethyleneoxy) ethanol,branched compound.

Other Types of Dispersants

Alternatively a surfactant or a mixture of surfactants with low HLBvalue (typically less than or equal to 8), preferably nonionic, or amixture of nonionics and ionics, may be used in the instant invention.

The dispersant for the oily liquid medium is a surfactant with lowhydrophile-lipophile balance (HLB) value (HLB <8) or a polymericdispersant of the type used in the lubricant industry. It is preferablynonionic, or a mixture of nonionics and ionics. A preferred dispersantfor the aqueous liquid medium is of high HLB value (HLB >10), preferablya nonylphenoxypoly(ethyleneoxy)ethanol-type surfactant. Of course, otheralcohol based glycols having a high HLB value can be used as well. Theuniform dispersion of nanotubes is obtained with a designed viscosity inthe liquid medium. The dispersion of nanotubes may be obtained in theform of a paste, gel or grease, in either a petroleum liquid medium oran aqueous medium.

The dispersants selected should be soluble or dispersible in the liquidmedium. The dispersant can be in a range of up from 0.01 to 30 percent,more preferably in a range of from between 0.5 percent to 25 percent,more preferably in a range of from between 1 to 20 percent, and mostpreferably in a range of from between 2 to 15 percent. The nanoparticlematerial can be of any desired weight percentage in a range of from0.001 up to 50 percent. For practical application it is usually in arange of from between 0.01 percent to 25 percent, and most preferably ina range of from between 0.1 percent to 20 percent. The remainder of theformula is the selected medium and other desired additives.

It is believed that in the instant invention the dispersant functions byadsorbing onto the surface of the nanoparticle material.

Other Chemical Compounds

This dispersion may also contain a large amount of one or more otherchemical compounds, preferably polymers, not for the purpose ofdispersing, but to achieve additional thickening or other desired fluidcharacteristics. These can be added but reduce the amount of particulatethat can be used without excessive thickening.

The viscosity improvers used in the lubricant industry can be used inthe instant invention for the oil medium for the purpose of achievingadditional thickening, which include olefin copolymers (OCP),polymethacrylates (PMA), hydrogenated styrene-diene (STD), andstyrene-polyester (STPE) polymers. Olefin copolymers are rubber-likematerials prepared from ethylene and propylene mixtures throughvanadium-based Ziegler-Natta catalysis. Styrene-diene polymers areproduced by anionic polymerization of styrene and butadiene or isoprene.Polymethacrylates are produced by free radical polymerization of alkylmethacrylates. Styrene-polyester polymers are prepared by firstco-polymerizing styrene and maleic anhydride and then esterifying theintermediate using a mixture of alcohols.

Other compounds which can be used in the instant invention in the oilmedium include: acrylic polymers such as polyacrylic acid and sodiumpolyacrylate, high-molecular-weight polymers of ethylene oxide such asPolyox® WSR from Union Carbide, cellulose compounds such ascarboxymethylcellulose, polyvinyl alcohol (PVA), polyvinyl pyrrolidone(PVP), xanthan gums and guar gums, polysaccharides, alkanolamides, aminesalts of polyamide such as Disparlon AQ series from King Industries,hydrophobically modified ethylene oxide urethane (e.g., Acrysol seriesfrom Rohmax), silicates, and fillers such as mica, silicas, cellulose,wood flour, clays (including organoclays) and nanoclays, and resinpolymers such as polyvinyl butyral resins, polyurethane resins, acrylicresins and epoxy resins.

Chemical compounds such as seal swell agents or plasticizers can also beused in the instant invention and may be selected from the groupincluding phthalate, adipates, sebacate esters, and more particularly:glyceryl tri(acetoxystearate), epoxidized soybean oil, epoxidizedlinseed oil, N,n-butyl benzene sulfonamide, aliphatic polyurethane,epoxidized soy oil, polyester glutarate, polyester glutarate,triethylene glycol caprate/caprylate, long chain alkyl ether, dialkyldiester glutarate, monomeric, polymer, and epoxy plasticizers, polyesterbased on adipic acid, hydrogenated dimer acid, distilled dimer acid,polymerized fatty acid trimer, ethyl ester of hydrolyzed collagen,isostearic acid and sorbian oleate and cocoyl hydrolyzed keratin,PPG-12/PEG-65 lanolin oil, dialkyl adipate, alkylaryl phosphate, alkyldiaryl phosphate, modified triaryl phosphate, triaryl phosphate, butylbenzyl phthalate, octyl benzyl phthalate. alkyl benzyl phthalate,dibutoxy ethoxy ethyl adipate, 2-ethylhexyldiphenyl phosphate, dibutoxyethoxy ethyl formyl, diisopropyl adipate, diisopropyl sebacate, isodecyloleate, neopentyl glycol dicaprate, neopenty giycol diotanoate, isohexylneopentanoate, ethoxylated lanolins, polyoxyethylene cholesterol,propoxylated (2 moles) lanolin alcohols, propoxylated lanoline alcohols,acetylated polyoxyethylene derivatives of lanoline, anddimethylpolysiloxane. Other plasticizers which may be substituted forand/or used with the above plasticizers including glycerine,polyethylene glycol, dibutyl phthalate, and2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and diisononylphthalate all of which are soluble in a solvent carrier. Other sealswelling agents such as LUBRIZOL 730 can also be used.

Antioxidants are an important part of transmission fluids. Generalclasses include zinc dialkyldithiophosphates, alkyl and aryl phenols,alkyl and aryl amines, and sulfuinzed olefins. Commercial examples areCIBA L57 (phenyl amine) and ETHYL HITEC 1656.

Pour point depressants, either of polymethyl methacrylate or ethylenepropylene olefin co-polymer type are useful to decrease the lowtemperature Brookfield viscosity of the ATF. Examples include ROHMAX3008, ROHMAX 1-333, LUBRIZOL 6662A.

Friction Modifiers are used to control friction and torquecharacteristics of the fluid. Commercial examples include LUBRIZOL 8650and HITEC 3191.

Physical Agitation

The physical mixing includes high shear mixing, such as with a highspeed mixer, homogenizers, microfluidizers, a Kady mill, a colloid mill,etc., high impact mixing, such as attritor, ball and pebble mill, etc.,and ultrasonication methods or passing through a small orifice such as afuel injector. Turbulent flows of any type will assist mixing.

Ball milling is the most preferred physical method in the instantinvention since it is effective at rapidly reducing particles to verysmall size while simultaneously dispersing them into a concentratedpaste as previously described. The concentrate can then be diluted withbase oil and other additives to hit a final target viscosity, dependingon the maximum temperature and shear conditions anticipated in thetarget vehicle application. For further size reduction and reducingparticle maximum size the diluted oil can be passed through a smallorifice such as a fuel injector. The raw material mixture may bepulverized by any suitable known dry or wet grinding method. Onegrinding method includes pulverizing the raw material mixture in thefluid mixture of the instant invention to obtain the concentrate, andthe pulverized product may then be dispersed further in a liquid mediumwith the aid of the dispersants described above. However, pulverizationor milling modifies the average aspect ratio of rod-like nanomaterials,e.g., carbon nanotubes. A detailed description has been given in anearlier section of the instant invention.

Ultrasonication is another physical method in the instant inventionsince it may be less destructive to the nanomaterial structure than theother methods described. Ultrasonication can be done either in thebath-type ultrasonicator, or by the horn-type ultrasonicator (or calledthe “wand”). More typically, horn-type ultrasonication is applied forhigher energy output. Sonication at the medium-high instrumentalintensity for up to 30 minutes, and usually in a range of from 10 to 20minutes is desired to achieve better homogeneity.

The instant method of forming a stable dispersion of nanomaterials in asolution consist of three steps. First select the appropriateconcentrate of dispersant or mixture of dispersing and other additivesfor the nanomaterial, and the oily medium, and dissolve the dispersantinto the liquid medium to form a concentrate solution (keeping in mindthe final additive concentrations desired following dilution); secondlyadd a high concentration of the nanomaterials, e.g., graphitenanoparticles or carbon nanotubes, into the dispersant-containingsolution, initiate strong agitation: ball milling, or ultrasonicating,or any combination of physical methods named; following an agitationtime of several hours, the resulting paste will be extremely stable andeasily dilutable into more base oils and additives to give the finaldesired concentrations of additives and the desired final viscosity.

EXAMPLES

Specific compositions, methods, or embodiments discussed are intended tobe only illustrative of the invention disclosed by this specification.Variation on these compositions, methods, or embodiments are readilyapparent to a person of skill in the art based upon the teachings ofthis specification and are therefore intended to be included as part ofthe inventions disclosed herein. Reference to documents made in thespecification is intended to result in such patents or literature citedare expressly incorporated herein by reference, including any patents orother literature references cited within such documents as if fully setforth in this specification.

An automatic transmission fluid D was formulation with the samecomposition as automatic transmission fluid (ATF) A and result is listin Example 1 as well. The particle size is measured by atomic forcemicroscopy (AFM), and FIG. 5 illustrates an AFM picture of ATF B. Thegraphite nanoparticles are plate-like structure, with average diameteris around 50 nm and thickness around 5 nm (as we described earlier,nanodisk or nanoplate).

Example 1 Automatic Transmission Fluids and Viscosity Data

ATF A B C D E* From Concentrate Paste A Paste B Paste C Paste D N/AKinematic viscosity at 7.55 19.68 10.83 7.48 7.15 100° C., cSt Kinematicviscosity at 28.44 29.32 28.77 27.85 33.67 40° C., cSt Viscosity Index254 634 395 257 183*E is an off-the-shelf regular commercial ATF (MERCON V).

Example 2 Engine Oil

Product Conventional Nanoparticle-containing Engine Oil Engine oilComposition Valvoline 90% DURABLEND 5W-30 DURABLEND 5W-30 9% DURASYN 1621% Graphite Process Bulk graphite is pulverized, and milled in DURASYN162 to obtain a paste. The paste is added to DURABLEND 5W-30 Viscosity10.66 10.90 @ 100° C. Viscosity 61.14 54.34 @ 40° C. Viscosity 166 197Index

Example 3 Shock Absorber Oil

Product Conventional Nanoparticle-containing Shock Oil Shock oilComposition VISTA LPA 62.40 77.70 210 LUBRIZOL 36.87 20.66 7720CLUBRIZOL 0.30 0.30 5186B Tricresyl 0.22 0.22 phosphate F-655C 0.20defoamer Blue Dye 0.01 0.01 Graphite 1.11 Process Graphite obtained aspowders (UCAR), and milled in VISTA LPA 210/LZ 7720C to obtain aconcentrate. Then the other ingredients are added to make the finalformulation Viscosity 8.97 7.77 @ 100° C. Viscosity 29.75 12.45 @ 40° C.Viscosity 307 732 Index

Example 4 Automatic Transmission Fluid (ATF)

Product Conventional Nanoparticle- Nanoparticle- Mercon V containingcontaining ATF ATF #1 ATF #2 Composition 2 cSt PAO 36.00 34.00 4 cSt PAO51.50 53.50 LUBRIZOL 10.50 10.50 ATF DI Package Graphite 2.00 2.00Process Graphite obtained Graphite foam as powders (UCAR), (POCO) wasand milled in pulverized, and 2 cSt PAO/DI milled in 2 cSt package toobtain PAO/DI package a concentrate. to obtain a Then other concentrate.ingredients are Then other added to make ingredients are the final addedto make formulation the final formulation Viscosity 7.70 7.57 7.37 @100° C. Viscosity 36.20 16.01 16.96 @ 40° C. Viscosity 190 527 475 Index

Example 5 Gear Lubricant

Product Conventional Nanoparticle-containing Gear Oil Gear OilComposition YUBASE 100N 47.70 4 cSt PAO 15.00 9.00 6 cSt PAO 67.00LUBRIZOL 10.00 10.00 Gear Oil DI Package LUBRIZOL 26.30 12.00 3174VISCOPLEX 1.00 1.00 0-112 Graphite 1.00 Process Graphite obtained aspowders (UCAR), and milled in 4 cSt PAO/DI package to obtain aconcentrate. Then the other ingredients are added to make the finalformulation Viscosity 14.21 14.79 @ 100° C. Viscosity 98.63 65.06 @ 40°C. Viscosity 148 240 Index

To demonstrate the temporary shear loss effect, a regular DEXRON III ATFand a nanofluid ATF were tested for high-temperature-high-shear (HTHS)viscosity, ASTM D 4683. This technique measures the high-temperature(150° C.) high-shear-rate viscosity of motor oils; very high shear rates(10⁶ s⁻¹) are obtained by using an extremely small gap between the rotorand stator wall. Low number means more temporary shear loss under thetest conditions.

Example 6 HTHS Viscosity of a DEXRON III ATF and a Nanofluid ATF

DEXRON III ATF Nanofluid ATF Graphite nanodisk 0 2% 100° C. KinematicViscosity 7.2 cSt 7.55 cSt HTHS Viscosity (150° C., 2.06 cP 1.74 cP 10⁶s⁻¹)

To demonstrate that there is no permanent shear loss to thesenanodisk-containing fluids, a standard European gear lubricant test, CECL-45-T-93, was run on a SYNPOWER 75W-90 and on a nanofluid gear oil.This test is designed to permanently shear down the non-shear-stablepolymers in the formulation through a special taper roller bearing rig.

Example 7 Permanent Shear Test Data on a SYNPOWER 75W-90 and a NanofluidGear Oil

SYNPOWER Nanofluid Gear 75W-90 Oil Graphite nanodisk 0   1%  100° C.14.90 cSt 18.14 cSt Kinematic Viscosity before shear 100° C. 13.96 cSt17.47 cSt Kinematic Viscosity after shear Percent Viscosity 6.31 3.69Loss due to shear

The foregoing detailed description is given primarily for clearness ofunderstanding and no unnecessary limitations are to be understoodtherefrom, for modification will become obvious to those skilled in theart upon reading this disclosure and may be made upon departing from thespirit of the invention and scope of the appended claims. Accordingly,this invention is not intended to be limited by the specificexemplification presented herein above. Rather, what is intended to becovered is within the spirit and scope of the appended claims.

1. The method of using nanomaterials as an effective viscosity modifierfor a lubricating oil, which provides better viscosity index, and withno adverse effect to the low temperature properties of the fluid, thanthe currently used polymer-based viscosity modifiers.
 2. The method ofclaim 1 in which the nanomaterials are made from carbon.
 3. The methodof claim 1 in which the nanomaterials are carbon nanotubes.
 4. Themethod of claim 1 in which the nanomaterials are graphite nanoparticles.5. The method of claim 3 wherein said carbon nanotube is eithersingle-walled, double-walled, or multi-walled, with typical aspect ratioof 100-2000.
 6. The method of claim 4 wherein said graphitenanoparticles are made by ball milling commercially available bulkgraphite or graphite powders or graphite foams into the desired particlesize, preferably with average diameter less than 500 nanometers.
 7. Themethod of claim 4 wherein said graphite nanoparticles are in the form ofnanodisks (or nanoplates).
 8. The method of claim 1 wherein saidnanomaterials are used in the final lubricating formulation in theweight percentage of from 0.001% to 50%, or more preferably from 0.01%to 25%, or more preferably from 0.1% to 20%.
 9. The method of claim 1 inwhich the said lubricating fluid can be engine oil for either gasolineor diesel engines, transmission fluid, gear oil, hydraulic fluid, shockabsorber oil, or any other lubricating fluid used in a modern vehicle.